A fluid that exhibits changes in viscosity under the influence of an electric field is referred to herein as "an electrorheological fluid". An "electrorheological response" is a phenomenon in which the rheology of a fluid is modified by the imposition of an electrical field. Electrorheological fluids have been known for several decades. A wide variety of such fluids are known in the art. They are also sometimes referred to as electroviscous fluids. It is generally known that electrorheological, or electroviscous, fluids exhibit pronounced resistance to shear, due to the changes in viscosity, in response to application of an electrical field.
Electrorheological fluids generally comprise suspensions of finely divided particles, often crystalline particles, that intentionally contain a certain amount of absorbed water. The suspensions are dispersions of such particles in an electrically non-conductive and non-polar liquid. The presence of the water in or on the dispersed particles has been generally acknowledged to be very important in achieving a significant change in viscosity under the influence of the applied electric field. For example, U.S. Pat. No. 3,047,507 Winslow teaches the addition of excess or absorbed water. In explaining mechanistically the role of the absorbed water, it is postulated that the presence of the absorbed water in or on the particulate material is necessary. It is described as necessary to promote ionization, and thus allow charges to move freely on the surface of the particles when an electric field is imposed.
Except for silica gels, and the like, prior ceramic particle dispersions were of finely divided crystalline particles. Silica gels can be considered to be an amorphous ceramic but they are highly hydrated. As indicated above, water in or on the ceramic particles, whether amorphous or crystalline, has been considered by many to be an important factor that influences magnitude of electrorheological effect. In other words, electroviscosity has been considered by many to be dependent upon water content in or on the dispersed finely divided ceramic particles. Various techniques have been proposed for controlling water content in prior art crystalline particulate materials.
One exception to the foregoing is the teachings of U.S. Pat. No. 4,744,914 Filisko et al. Filisko et al. teach that water content in a crystalline material varies with temperature, and that this variability can provide a variable electrorheological response. Filisko et al. propose an electrorheological fluid having a dispersed phase of a particular crystalline zeolite that is substantially free of adsorbed water. The suspending dielectric fluid is dry, as well as the suspended particles. Hence, little or no water can be lost when the suspension is used above room temperature. Accordingly, the Filisko et al. electrorheological fluid is more stable during use at elevated temperatures. This is particularly important in the automobile industry, which generally requires products to be stable over a temperature range of about -40.degree. to +140.degree. C.
Zeolites are a particular crystalline form of aluminosilicates. However, heretofore, the electrorheological advantages of using anhydrous amorphous ceramic particles as the dispersed phase in an electrorheological fluid have not been recognized. It appears that there may even be special advantages to be obtained with anhydrous amorphous particles produced from a gel or solgel that is rapidly dried. Amorphous materials are not limited to only those compositions which will precipitate or solidify into a crystalline form. Amorphous materials can thus have virtually any composition. This opens the door to the investigation of a wide variety of synthetic ceramic compositions for enhanced electrorheological effects. Even though this recognition is new, enhanced electrorheological effects have already been found. However, it is believed that the work done in this connection is only beginning. This invention makes available the opportunity to very precisely "tailor" the composition of the dispersed particles. Results obtained to date indicate that even more electrorheologically effective anhydrous amorphous materials, and/or anhydrous amorphous material/dielectric fluid combinations, may be found in the future.
As indicated, we have found that dispersed particles of many amorphous ceramic compositions exhibit significant electrorheological response even when substantially free of water. Thus, like the crystalline zeolites disclosed in U.S. Pat. No. 4,744,914 Filisko et al., anhydrous amorphous ceramics can be used in electrorheological fluids at elevated temperatures. This makes them useful in a significant wide variety of applications.
Still further, it is to be recognized that dry materials have a natural tendency to eventually absorb, or re-absorb, water to some extent. However, we have found that anhydrous amorphous ceramic compositions have a decidedly lesser tendency to adsorb, or re-absorb, water than their crystalline counterparts. This can be a very important attribute. Absorption of water by the dispersed particles in an electrorheological fluid can cause the fluid to change its electrorheological response. In other words, the response of the fluid is not stable over time. This is a durability problem. In some applications, as for example automotive applications, long durability is of significant concern. In that sense, this invention can be considered to be a specific improvement on the concepts taught in U.S. Pat. No. 4,744,914 Filisko et al.
A wide variety of substantially dry amorphous ceramic compositions will apparently exhibit a significant electrorheological response. This makes them inherently more useful in a wider variety of applications, including automotive applications and other elevated temperature applications.
Still another attribute of this invention may be realized in a particular method of recovering amorphous particles from a gel or solgel in which they are formed. Tests made thus far indicate that selected compositions of pyrolytically dried gel or solgels provide amorphous ceramic particles of significantly enhanced electrorheological response.